Skin impedance

Question 1

Describe Ohm’s law for AC and DC circuits.

Answer 1

in case of DC circuits: U = I * R
voltage proportional to the current intensity

in case of AC circuits: U_rms = I_rms * Z
root-mean-square (rms, i.e. effective) voltage proportional to the rms current

Question 2

What is resistance?

Formula + unit.

Answer 2

ability to resist the electric current
(has only magnitude)
_{→ R inversely proportional to A}

R = ρ * l/A
in ohm = [Ω]

• ρ = specific resistance of the material
• l = length
• A = cross sectional area of the conductor

_{(REMEMBER: wider wires have a greater cross-sectional area. Water will flow through a wider pipe at a higher rate than it will flow through a narrow pipe. This can be attributed to the lower amount of resistance that is present in the wider pipe. In the same manner, the wider the wire, the less resistance that there will be to the flow of electric charge. When all other variables are the same, charge will flow at higher rates through wider wires with greater cross-sectional areas than through thinner wires)}

Question 3

What is a capacitor?

Answer 3

2 conducting plates seperated by an insulator layer

Question 4

What is capacity?

Formula + unit.

Answer 4

electric charge collecting ability of a capacitor

C = ε * (A/l)
in farad = [F]

• ε = dielectric constant
• A = surface area of the plate
• l = distance btw the plates

_{(same formula as for R, only constant is changed)}

Question 5

What is capacitive reactance?

Formula + unit.

Answer 5

internal resistance of a capacitor in AC circuits

X_C = 1/2π * f * C
in ohm = [Ω]

• f = frequency of AC
• C = capacity

Question 6

Which components were used for the measurement of skin impedance?

Answer 6

• arbitrary waveform generator: source of AC/DC current
• digital multimeter: ammeter
• auxiliary + measuring electrode
• hand obv.

Question 7

Explain the simplified model of the skin impedance.

Differentiate the flow of the current in AC and DC circuits.

Answer 7

ohmic resistance (R) and capacitive reactance (X_C) connected in parallel circuit

• in DC circuit: I flows through resistances → only R relevant
• in high frequency AC circuit: I flows through capacitors → only X_C relevant

Question 8

What is the equivalent resistance of resistors connected in series?

Answer 8

sum of individual resistances

R_e = R₁ + R₂ + …

Question 9

What is impedance?

Formulas + unit.

Answer 9

total resistance in AC circuits

Therefore:

• Z = U_{gen rms}/I_{skin rms} (rearranged Ohm’s law, R substituted by Z)
• Z_e = X_C1 + X_C2 + … (if connected in series)

in ohm = [Ω]

(magnitude + phase since direction of current flow changes)

Question 10

What is the specific resistance of the skin?

Formula + units.

Answer 10

resistance of the unit area of the skin

ρ* = R * A
in [Ωm²] or [kΩcm²]

_{(NOTICE: specific resistance = R<strong>*A</strong>, specific capacitance = C<strong>/A</strong>)}

Question 11

What is the specific capacitance of the skin?

Formula + units.

Answer 11

capacitance of the unit area of skin

γ* = C/A
in [F/m²] or [μF/cm²]

_{(NOTICE: specific resistance = R<strong>*A</strong>, specific capacitance = C<strong>/A)</strong>}

Question 12

List some medical applications of impedance measurement.

Answer 12

• impedance tomography: resistance map constructed by large no. of applied electrodes on a body part (e.g. for vascular stenosis, pulmonary edema)
• impedance cardiography: heart is mapped, stroke volume can be measured as a time change of the resistance map
• impedance plethysmography: blood supply of limbs mapped